Light Scattering Measurements of KCl Particles as an Exoplanet Cloud Analog

TL;DR

This study introduces a new apparatus, ExCESS, to measure light scattering of KCl cloud analogs, revealing that traditional Lorenz-Mie calculations inaccurately predict scattering for non-spherical particles relevant to exoplanet atmospheres.

Contribution

The paper presents the ExCESS system for measuring scattering properties of cloud particles and demonstrates the limitations of Lorenz-Mie theory for non-spherical KCl particles in exoplanet cloud modeling.

Findings

Lorenz-Mie overestimates backscattering intensity.

Lorenz-Mie incorrectly predicts scattering at mid-phase angles.

Non-spherical particles have distinct scattering properties.

Abstract

Salt clouds are predicted to be common on warm exoplanets, but their optical properties are uncertain. The Exoplanet Cloud Ensemble Scattering System (ExCESS), a new apparatus to measure the scattering intensity and degree of linear polarization (DOLP) for an ensemble of particles, is introduced here and used to study the light scattering properties of KCl cloud analogs. ExCESS illuminates particles with a polarized laser beam (532 nm) and uses a photomultiplier tube detector to sweep the plane of illumination. Scattering measurements for KCl particles were collected for three size distributions representative of modeled clouds for the warm exoplanet GJ 1214b. Our measurements show that Lorenz-Mie calculations, commonly used to estimate the light scattering properties of assumedly spherical cloud particles, offer an inaccurate depiction of cubic and cuboid shaped KCl particles. All of our measurements indicate that Lorenz-Mie scattering overestimates the backscattering intensity of our cloud analogs and incorrectly predicts the scattering at mid-phase angles (~90 degrees) and preferential polarization state of KCl scattered light. Our results align with the general scattering properties of non-spherical particles and underscore the importance of further understanding the effects that such particles will have on radiative transfer models of exoplanet atmospheres and reflected light observations of exoplanets by the upcoming Nancy Grace Roman Space Telescope and Habitable Worlds Observatory.